A CDMA system has advantages of large capacity, high service quality and good security etc., so it has become one of the main techniques for development of the third generation mobile communication. Multiple Address Interference (MAI) limits capacity and performance of a CDMA system. Usually, MAI of a certain user is small to other users, but if the number of users is great, the total MAI caused by other users is large. When near-far effect exists, a larger amplitude signal of a certain user makes larger MAI to a user with a weaker signal. In some cases the weaker and useful signal is flooded by the stronger MAI.
Multi-user detection is an enhanced technique to overcome MAI and to raise capacity of a CDMA system. Joint detection of multi-user is made to reduce influence of MAI on performance of a receiver and to raise the system capacity as much as possible.
In multi-user detection technique, the method of parallel interference cancellation takes the expected user signal as a useful signal and signals of other users as interference signals; interference signals of all other users are removed parallelly from the received signal for each user to obtain the expected user signal, and then the expected user signal is detected, in this way the system performance is raised.
FIG. 1 shows a diagram of a traditional receiver with Parallel Interference Cancellation (PIC). The 101 part of the PIC structure includes an MAI estimation and interference cancellation device and N user processing units shown in FIG. 2, wherein N is the number of parallel processing users, i.e. each user corresponds to one user processing unit shown in FIG. 2, respectively. The final stage of the PIC structure 102 includes N processing units shown in FIG. 3.
First stage of the PIC structure 101 takes the baseband signal of the received signal as an input signal of every user; the input signal is processed and an output signal of every user is obtained; the output signal is the next stage input signal of the PIC structure for every user. Second stage of the PIC structure processes the first stage output signal of every user and obtains an output signal of every user that is the input signal of next stage. The process goes on stage-by-stage until the final stage 102, and the output signal of the final stage is the final result of multiple stages of the PIC structure for every user.
Take the user processing-unit as an example, as shown in FIG. 2. The RAKE receiver 103 makes multipath de-spread of an input signal, and takes the de-spread result to make the channel estimation, and then makes multipath combination. The multipath combination result of the RAKE is sent to the hard decision maker 104, and the channel estimation result is sent to the signal regenerator 105. The hard decision maker 104 makes decision for the multipath combination result and sends the decision result to the signal regenerator 105. According to the results of the channel estimation and the decision, the signal regenerator 105 regenerates a signal of the user and sends it to the MAI estimation and interference cancellation device 106. The MAI estimation and interference cancellation device 106 accumulates regenerated signals of other users and calculates the MAI of the user, and then removes the MAI from the received baseband signal to obtain a result that is an input of the user RAKE receiver at the next stage of the PIC structure.
FIG. 3 is a diagram of the final stage of the PIC structure in FIG. 1. Take one user processing-unit as an example. The RAKE receiver of the user 107 de-spreads the input signal; also the RAKE receiver 107 makes channel estimation and multipath combination to obtain the soft output of the user. The soft output of a user is the final result of the multi-stage PIC structure. In a receiver, the soft output of a user is sent to a user decoder for decoding.
There are two methods to improve the traditional parallel interference cancellation.
First method is called a double-layer weighting parallel interference cancellation method, as shown in FIG. 4, that includes multiple stages of PIC structure 201 and a last stage of PIC structure 203. Each PIC structure 201 includes a MAI estimation and cancellation device and N user processing units, as shown in FIG. 5; the final stage PIC structure 203 includes N processing units, as shown in FIG. 3.
FIG. 5 shows a user processing-unit of the double-layer weighting parallel interference cancellation method. In FIG. 5, the RAKE receiver 204 makes multipath de-spread for an input signal, and makes channel estimation with the de-spread result, and then makes multipath combination. The multipath combination result of the RAKE receiver is sent to the hard decision maker 205 and the reliability coefficient generator 208 simultaneously, and the channel estimation result is sent to the reliability coefficient generator 208 and the signal regenerator 206, simultaneously. The hard decision maker 205 makes a decision for the multipath combination result and sends the decision result to the signal regenerator 206. Based on the channel estimation result and the multipath combination result, the reliability coefficient generator 208 calculates reliability coefficients for the decision result made by the hard decision maker, and sends the reliability coefficients to the signal regenerator 206. Based on the decision result, the reliability coefficients and the channel estimation result, the signal regenerator 206 generates a regenerated signal and sends it to the MAI estimation and interference cancellation device 207. The MAI estimation and interference cancellation device 207 accumulates the regenerated signals of other users and calculates the user MAI, and then the user MAI is partly removed from the baseband signal of the user received signal to obtain an input signal for the RAKE receiver of next stage.
The final stage of PIC structure of this improved method is same as the final stage of the traditional PIC method shown in FIG. 3.
The double-layer weighting parallel interference cancellation method guarantees that the decision cost is minimum, and at the same time it compensates statistical deviation of the user signal estimation through part MAI cancellation.
The second method is called a parallel interference cancellation method with a front-back stage combiner.
The multiple stage structure of a parallel interference cancellation method with a front-back stage combiner is shown in FIG. 6, and it includes: first stage 301, the middle stages 302 and last stage 303. In FIG. 6, first stage PIC structure 301 includes: a MAI estimation and interference cancellation device and N user processing units shown in FIG. 7, wherein N is the number of the parallel processed users; a middle stage 302 includes: a MAI estimation and interference cancellation device and N user processing units shown in FIG. 8; and last stage 303 includes N processing units shown in FIG. 9.
Taking a user processing-unit as an example, FIG. 7 is a diagram of the first stage of the PIC structure. In FIG. 7, the RAKE receiver 304 makes multipath de-spread for an input signal, and makes the channel estimation with the de-spread result, and then makes multipath combination; the result of multipath combination is sent to the hard decision maker 305 and the result of channel estimation is sent to the signal regenerator 306. The hard decision maker 305 makes a decision for the multipath combination result and sends the decision result to the signal regenerator 306. Based on the decision result and the channel estimation result, the signal regenerator 306 generates a regenerated signal and sends the regenerated signal to the MAI estimation and interference cancellation device 307. The MAI estimation and interference cancellation device 307 accumulates the regenerated signals of other users and calculates the user MAI, and then the user MAI is removed from the baseband signal of the user received signal to obtain an input signal for the RAKE receiver of the next stage of the PIC structure. In this stage, the multipath combination result of the user and the noise power of the multipath combination result are exported to the front-back stage combiner of the user in next stage.
FIG. 8 shows a middle stage of the PIC method with a front-back stage combiner. In FIG. 8, the RAKE receiver 308 makes multipath de-spread for an input signal, and makes the channel estimation with the de-spread result, and then makes multipath combination. The multipath combination result of the RAKE receiver is sent to the front-back stage combiner 309, and the channel estimation result is sent to the signal regenerator 311. The multipath combination result and noise power from last stage (the stage before this stage) are also sent to the front-back stage combiner 309. The front-back stage combiner 309 first takes the noise power of this stage multipath combination result and the noise power exported from last stage to obtain a combination coefficient, and then according to the combination coefficients the front-back stage combiner combines proportionally the multipath combination result exported from last stage and the multipath combination of this stage; the combined result is sent to the hard decision maker 310. At the same time, said combined result and the noise power of this combined result are sent to the front-back stage combiner of the next stage of this user. The hard decision maker 310 makes decision with said combined result and sends a decision result to the signal regenerator 311; with the decision result and the channel estimation result the signal regenerator 311 generates a regenerated signal and sends the regenerated signal to the MAI estimation and interference cancellation device 312. The MAI estimation and interference cancellation device 312 accumulates the regenerated signals of other users and calculates the user MAI, and then the user MAI is removed from the baseband signal of the user received signal to obtain an input signal for the RAKE receiver of the next stage of the PIC structure.
FIG. 9 shows the final stage of the PIC method with a front-back stage combiner. In FIG. 9, the RAKE receiver 313 makes multipath de-spread for an input signal, and makes the channel estimation with the de-spread result, and then makes multipath combination. The multipath combination result of the RAKE receiver is sent to the front-back stage combiner 314, and the combined result and noise power of the stage that before the final stage and the noise power of the final stage are imported to the front-back stage combiner 314 too. The front-back stage combiner 314 first takes the noise power of this stage and the noise power exported from last stage to obtain a combination coefficient, and then according to the combination coefficient the front-back stage combiner combines proportionally the multipath combination result exported from last stage and the multipath combination result of this stage; the combined result is the final result of the multi-stage PIC structure. In a receiver, the final result is sent to a decoder for decoding.
According to the criteria of maximizing SNR of the combined result, the optimal combination coefficient can be obtained. Nevertheless, calculation of the optimal combination coefficient is very complicated; it is necessary to use the noise power and signal amplitude exported from last stage, the noise power and signal amplitude of the RAKE multipath combination result of this stage, and the correlation coefficient of the noise involved in the RAKE multipath combination result of this stage and the noise involved in the RAKE multipath combination result of last stage.
In order to make a parallel interference cancellation method with a front-back stage combiner easier to implement, usually it is supposed that the noise involved in the RAKE multipath combination result of this stage and the noise exported from last stage are noncoherent. The less optimal combination coefficient is calculated by the noise power of noise exported by last stage and the noise power of the RAKE multipath combination result of this stage.
The parallel interference cancellation method with a front-back stage combiner mentioned above raises performance of the traditional parallel interference cancellation method, but the assumption that noises are noncoherent between two consecutive stages limits performance to be raised further.